Dispatch non-trunked communication systems are known. Such systems typically consist of a central repeater and a number of mobile communication units operating on a pair of frequencies over a relatively large geographic area. The communication units using such a system occasionally belong to a single operating group but more often belong to a number of groups.
When a member of a group desires use of such a system he keys his microphone and begins speaking. If a second user were to desire use of the system he must wait for the first user to finish. If the second user were to begin speaking without waiting for the first user to finish there would be an interference of signals. The strongest signal received at the repeater would be re-transmitted. The weaker signal would be lost.
Targets of such transmissions normally receive transmissions only from other members of the same group. Targets recognize transmitted messages from other members through the inclusion of an identification tone or code transmitted along with the signal.
The receiver within a target communication unit is constructed to recognize specific identification tones or codes transmitted from other members his group and to be able to discriminate between that tone and tones assigned to other groups. A receiver will activate (un-mute) only upon receipt of the tone or code assigned to his group.
The tone assigned to a group typically consists of a frequency below three-hundred hertz. Communication units filter the tone out of the reproduced audio signal. The result is that a target unit activates on receipt of the transmitted tone but the target user does not hear the tone.
In use a target user will hear only transmissions from other members of the his group. As such a user may not even be aware of use of the system by other groups. If a user, on the other hand, should desire to use the system (transmit a message), then use by other groups must be considered and a call potentially delayed. For the former user to detect other users he must first manually un-mute his radio and listen for other users. If the system is not being used then he may transmit. If, in the interim, another group has begun transmitting then the former user must wait.
Problems often arise on such a system because of an inability of users to anticipate or detect other users. A transmission by one member of a group may require a response from another member of the group. The other member, on the other hand, may be busy or may be slow in providing a response. Because of the delay another user, thinking that the previous transaction has ended, may begin transmitting. If the previous target, should now respond the result is contention for use of the system. Where contention exists for channel use the result is often missed or lost messages.
Another problem arises in the lost time spent monitoring the system by potential users. During times of heavy use considerable time may be consumed waiting for a clear channel.
Further problems may arise due to the lack of privacy of such systems. Occasionally information of a sensitive nature must be communicated to members of a group. Because other groups must monitor the system to affect usage a transmitting party is never certain who receives the transmitted information.
Because of the inherent problems and inefficiencies associated with the use of non-trunked dispatching systems a better method is needed to control resource allocations between talk groups.